WASHINGTON (Reuters) - The United States prepared to step up sanctions against Moscow if pro-Russian military actions in eastern Ukraine continue, a senior U.S. envoy said on Sunday, but it is unclear whether new measures will win European support or slow the Russian-backed separatist revolt.

The next round of U.S. sanctions, which would be the fourth imposed since the Ukraine crisis began, is likely to target Russians close to Putin as well as Russian entities, three sources familiar with the discussions said on Sunday.

However, they will not necessarily target entire Russian business sectors such as mining, banking and energy, the sources said, adding that the situation was fluid.

"Broad sectoral sanctions are probably not up for immediate action. What is up for immediate action is ... (Russian) individuals and entities," said one of the sources. "All of this is subject to what is happening on the ground."

Even if the United States were to target swathes of Russian industry, U.S. officials said it was uncertain whether Europe would go along with penalties that would affect Russia's economy and powerful energy sector.

The sanctions have been the most visible sign of U.S. anger at Russia's annexation of the Crimea region in southern Ukraine last month, reflecting the deepest plunge in U.S.-Russian relations since the Cold War.

Pro-Russian activists seized government buildings on Saturday in the eastern town of Slaviansk, about 150 km (90 miles) from the Russian border. Ukrainian security forces were trying to oust the activists, who set up barricades on the outskirts of the city.

The American ambassador to the U.N., Samantha Power, said on ABC's "This Week" that the latest events in Ukraine bore "the telltale signs of Moscow's involvement."

"The president has made clear that, depending on Russian behaviour, sectoral sanctions in energy, banking, mining could be on the table, and there's a lot in between," Power said.

"I think we've seen that the sanctions can bite, and if actions like the kind we've seen over the last few days continue, you're going to see a ramping up of those sanctions."

Power said sanctions already imposed by Washington have had an impact: the Russian rouble has fallen to an all-time low, the country's stock market has depreciated by 20 percent and investors are fleeing the country.

But beyond declines in the rouble and Russian indexes, the impact has been modest. Most of those hit by U.S. and EU sanctions are not known to have extensive business interests.

One of those who does is billionaire oil and gas trader Gennady Timchenko. He told Russian television on Saturday his inclusion on the U.S. sanctions list had caused minor problems, mainly because some European banks had become wary of carrying out transactions with any entities linked to him.

But he said, for him personally, being on the list was "quite an honour.

A new round of U.S. sanctions against Russia is likely to target influential people or firms in its business sectors, such as energy, engineering and financial services, as spelled out in President Barack Obama's executive order last month.

The U.S. goal is to put more pressure on Russian President Vladimir Putin's business allies. When sanctions were announced in March, the U.S. Treasury said it would largely focus on people and their personal assets, not businesses they operate.

But U.S. officials, speaking on condition of anonymity, said there was no certainty European nations would be on board with sanctions that would cause real pain and be felt not only in Russia's economy but by U.S. and European interests too.

Broader sanctions could cause collateral damage to companies that would be forced to sever ties with blacklisted firms and could also affect Russian energy deliveries to western Europe.

It was also unclear whether Thursday's planned Geneva talks involving the United States, Russia and Ukraine would happen now, the U.S. officials said, as the Russians had begun setting preconditions, including that Ukraine could not use force to quell unrest in the east, and that all of Ukraine's regions, including pro-Russian areas, be included in the discussion.

The three sources said the Obama administration had been hesitant to impose the sectoral sanctions but that events over the weekend in eastern Ukraine had clearly tilted the debate in the direction of doing more, rather than less.

European Union foreign ministers are scheduled to meet on Monday to discuss further Ukraine sanctions. One source said the United States would likely hold off on deciding on further sanctions until it sees where the EU consensus emerges.

The three sources familiar with the debate said there were divergences within the European Union, with some advocating sectoral sanctions, others for targeting more individuals and entities, and others preferring to do nothing before Thursday's meeting of U.S., Russian, EU and Ukrainian officials in Geneva.

"I don't think we'll see tier-three tomorrow, but it's very difficult to envisage not seeing anything after the events of the weekend," said another of the three sources, referring to broad sectoral sanctions.

DESIGNS ON EASTERN UKRAINE?

Ukraine now faces a rash of rebellions in the east it says are inspired and directed by the Kremlin.

Asked on ABC if Putin wants to seize eastern Ukraine, Power said his actions "give credence to the idea."

Though Russians are insisting that is not what Moscow wants, she said, "Everything they're doing suggests the opposite."

NATO described the appearance in eastern Ukraine of men with specialized Russian weapons and identical uniforms without insignia, as previously worn by Moscow's troops when they seized Crimea, as a "grave development."

Power said the rebellion has "all the telltale signs of what we saw in Crimea: It's professional, it's coordinated, there's nothing grassroots-seeming about it. The forces are doing in each of the six or seven cities that they've been active in exactly the same thing."

However, the U.S. officials who spoke on condition of anonymity said Russia's recent moves appeared to be less about grabbing territory than about destabilizing what Russia sees as an increasingly Western-centric government in Kiev.

The United States on Friday imposed sanctions on a Crimea-based gas company, Chernomorneftegaz, effectively putting it off limits to Russia's state-controlled Gazprom, which was expected to bid for a stake in the company.

The move, along with penalties on six Crimean separatists and a former Ukrainian official, is the third round of U.S. sanctions since the Ukraine crisis erupted.

KIEV/UNITED NATIONS (Reuters) - The United Nations Security Council held an emergency session on Sunday night to discuss the escalating crisis in Ukraine, just hours before a deadline by Kiev for pro-Russian separatists to disarm by Monday morning or face a "full-scale anti-terrorist operation" by its armed forces.

The Council began meeting at 8 p.m. (01.00 a.m. BST on Monday) at Russia's request after Moscow called Kiev's plans to mobilise the army to put down a rebellion by pro-Russian militants in eastern Ukraine "criminal."

Britain's U.N. ambassador said Russia had massed tens of thousands of well-equipped troops near the Ukrainian border in addition to the 25,000 troops it recently moved into Crimea, which Moscow seized last month.

"Satellite images show that there are between 35,000 and 40,000 Russian troops in the vicinity of the border with Ukraine equipped with combat aircraft, tanks, artillery and logistical support units," Ambassador Mark Lyall Grant said.

"This is in addition to the 25,000 Russia troops based illegally in Crimea," Lyall Grant added in his speech during the U.N. emergency meeting.

Angered by the death of a state security officer and the wounding of two comrades near the flashpoint eastern city of Slaviansk, Ukrainian acting president Oleksander Turchinov gave rebels occupying state buildings until 0600 GMT to lay down their weapons.

"The National Security and Defence Council has decided to launch a full-scale anti-terrorist operation involving the armed forces of Ukraine," Turchinov said in an address to the nation.

He blamed Russia, which annexed Ukraine's Crimea region when Moscow-backed former president Viktor Yanukovich fled after months of pro-Western protests, for being behind the rash of rebellions across Russian-speaking towns in eastern Ukraine.

"We will not allow Russia to repeat the Crimean scenario in the eastern regions of Ukraine," Turchinov said.

The deadline and the standoff with Russian troops at the border have raised fears of a military confrontation with Moscow.

The head of Ukraine's state security service (SBU) said government forces would respond ruthlessly if pro-Russian separatists opened fire.

"If they open fire, we will annihilate them. There should be no doubt about this," Valentyn Nalyvaichenko said in a televised interview.

Russia's Foreign Ministry called the planned military operation a "criminal order" and said the West should bring its allies in Ukraine's government under control.

"It is now the West's responsibility to prevent civil war in Ukraine," the ministry said in a statement.

The 15-nation council has held numerous emergency meetings on Ukraine but has been incapable of taking any concrete action because of Russia's sharp disagreements with the United States and Europe.

Earlier, the American ambassador to the U.N., Samantha Power, said on ABC's "This Week" that the United States was prepared to step up sanctions against Moscow if pro-Russian military actions in eastern Ukraine continued.

"The president has made clear that, depending on Russian behaviour, sectoral sanctions in energy, banking, mining could be on the table, and there's a lot in between," she added.

Ukraine has repeatedly said the rebellions are inspired and directed by the Kremlin. But action to dislodge the armed militants risks tipping the stand-off into a new, dangerous phase as Moscow has warned it will protect the region's Russian-speakers if they come under attack.

One Ukrainian state security officer was killed and five were wounded on the government side in Sunday's operation in Slaviansk, Interior Minister Arsen Avakov said. "There were dead and wounded on both sides," he wrote on his Facebook page.

WELL ORGANISED ATTACKERS

The separatists are holed up in the local headquarters of the police and of the state security service, while others have erected road blocks around Slaviansk, about 150 km (90 miles) from the Russian border.

Kiev accuses the Kremlin of trying to undermine the legitimacy of presidential elections on May 25 that aim to set Ukraine back on a normal path after months of turmoil.

However, Russian Foreign Minister Sergei Lavrov said Kiev was "demonstrating its inability to take responsibility for the fate of the country" and warned that any use of force against Russian speakers "would undermine the potential for cooperation", including talks due to be held on Thursday between Russia, Ukraine, the United States and the European Union.

Relations between Russia and the West are at their worst since the Cold War, due to the crisis that began when Moscow-backed Yanukovich was pushed out by popular protests in February.

Moscow then annexed Crimea from Ukraine, saying the Russian population there was under threat. Some Western governments believe the Kremlin is preparing a similar scenario for eastern Ukraine, something Moscow has strenuously denied.

In Kramatorsk, about 15 km (9 miles) south of Slaviansk, gunmen seized the police headquarters after a shootout with police, a Reuters witness said.

The attackers were a well-organised unit of more than 20 men, wearing matching military fatigues and carrying automatic weapons, who had arrived by bus. Video footage showed the men taking orders from a commander. Their identity was unclear.

Their level of discipline and equipment was in contrast to the groups which have occupied buildings so far in Ukraine. They have been mostly civilians formed into informal militias with mismatched uniforms.

In Washington, the U.S. State Department said pro-Russian militants seizing government buildings in six cities in eastern Ukraine on Saturday was an orchestrated operation reminiscent of those conducted in Crimea before it was annexed by Russia.

"Many of the militants were outfitted in bullet-proof vests and camouflage uniforms with insignia removed and carrying Russian-origin weapons," it said in a note entitled "Evidence of Russian Support for Destabilization of Ukraine."

"These operations bear many similarities to those that were carried out in Crimea in late February and culminated in Russia's illegal military intervention and purported annexation of Crimea," the State Department note said.

NATO Secretary-General Anders Fogh Rasmussen also expressed concern about similarities in the appearance of some rebels to that of the Russian troops who seized control in Crimea.

Calling on Russia to pull back its large number of troops, including special forces, from the area around Ukraine's border, he said in a statement: "Any further Russian military interference, under any pretext, will only deepen Russia's international isolation."

NATO has effectively ruled out military action over Ukraine, which lies outside the Western alliance. However, Washington and NATO leaders have made clear they would defend all 28 member states, including former Soviet republics in the Baltic that are seen as the most vulnerable to Russian pressure.

NATO allies have beefed up their air and sea firepower in eastern Europe. The alliance has also cut off cooperation with Russia and stepped up work with Ukraine, including advising its military on reforms and promising to increase joint exercises.

With EU foreign ministers due to discuss the crisis in Luxembourg on Monday, Britain called on Moscow to disown the rebels.

The crisis over Ukraine could trigger a "gas war", disrupting supplies of Russian natural gas to customers across Europe. Moscow has said it may be forced to sever deliveries to Ukraine - the transit route for much of Europe's gas - unless Kiev settles its debts.

For now, though, the focus of the crisis is in eastern Ukraine, the country's industrial heartland, where many people feel a close affinity with neighbouring Russia.

(Additional reporting by Thomas Grove in Slaviansk, Ukraine, Natalia Zinets in Kiev, Alessandra Prentice in Moscow, William James in London, Adrian Croft in Brussels and Lour Charbonneau at the United Nations; Writing by Christian Lowe, Richard Balmforth, David Stamp and Sandra Maler; Editing by Clarence Fernandez)

SYDNEY/KUALA LUMPUR (Reuters) - Australian officials leading the search for a missing Malaysia Airlines plane in the southern Indian Ocean are weighing when to deploy an underwater robot to aid in the hunt, now in its sixth fruitless week.

Malaysia Airlines Flight MH370 disappeared soon after taking off on March 8 from Kuala Lumpur bound for Beijing with 227 passengers and 12 crew on board, triggering a multinational search that is now focused on the Indian Ocean.

Searchers are confident they know the approximate position of wreckage of the Boeing 777, some 1,550 km (963 miles) northwest of Perth, after picking up several acoustic signals they believe are from its black box recorders.

With the batteries on the locators now two weeks past their 30-day expected life, the focus may soon shift to trying to find wreckage using sonar and cameras on a small unmanned "robot" known as an Autonomous Underwater Vehicle (AUV).

Australia's Ocean Shield, towing a sophisticated U.S. Navy "towed pinger locator", and Britain's HMS Echo are still criss-crossing the zone where four signals or pings were picked up, but the last was almost a week ago.

"This work continues in an effort to narrow the underwater search area for when the Autonomous Underwater Vehicle is deployed," the Australian agency heading the search said in a statement on Sunday.

Up to a dozen planes and 15 ships will be searching in three separate areas on Monday, the furthest some 2,250 km (1,400 miles) from Perth, the agency added.

The AUV onboard the Ocean Shield, called a Blue-fin 21, could take months to scan and map the 600 sq km (230 sq miles) zone currently the focus of the acoustic search - an area the size of a medium city.

"Trying to locate anything 4.5 kilometres beneath the surface of the ocean about 1,000 kilometres from land is a massive, massive task and it is likely to continue for a long time to come," Australian Prime Minister Tony Abbott said at the weekend.

CO-PILOT CALL?

The mystery has sparked what is on track to be the most difficult and expensive search and recovery operation in aviation history.

An aircraft's black box records data from the cockpit and conversations among flight crew and may provide answers about what happened to the missing plane, which flew thousands of kilometres (miles) off course after taking off.

Malaysia is focusing its criminal investigation on the cabin crew and the pilots of the plane -- 53-year-old captain Zaharie Ahmad Shah and 27-year old Fariq Abdul Hamid -- after clearing all 227 passengers of any involvement, police have said.

Malaysia's acting Transport Minister Hishammuddin Hussein appeared to hose down a weekend report that investigators suspect that the co-pilot tried to make a call with his cellphone after the plane was diverted from its scheduled route.

Malaysia's government has also begun investigating civil aviation and military authorities to determine why opportunities to identify and track the flight were missed in the chaotic hours after it vanished.

(Editing by Michael Perry)

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